The lead-acid battery includes, in a battery container, an electrode plate group immersed in an electrolyte, and the opening of the battery container is hermetically sealed with a battery container cover. The electrode plate group includes a stack of a positive electrode plate, a negative electrode plate, and a separator interposed between the positive electrode plate and the negative electrode plate.
The negative electrode plate includes a negative electrode grid, and a negative electrode active material held by the negative electrode grid. Usually, an organic anti-shrink agent such as lignin is added to the negative electrode active material in order to achieve a prolonged life and an enhanced discharge performance at low temperatures. In general, the organic anti-shrink agent contains a sodium salt, so that sodium ions in an amount of several hundreds of mmol/L is dissolved in the electrolyte.
With repeated use of the lead-acid battery, the moisture in the electrolyte is gradually decreased owing to electrolysis and the like. To compensate for this moisture loss, an appropriate amount of water is added.
Since the lead-acid battery is heavily used, for example, for starting an automobile engine, vehicle maintenance workers often perform adding water for the lead-acid battery at the time of a regular vehicle inspection, which is conducted every two years.
A liquid port for adding water is provided in the battery container cover of the lead-acid battery. Normally, the liquid port is sealed with a liquid port plug. During adding water, the liquid port plug is removed, and water is added from the liquid port. A person who performs adding water adds water while visually recognizing the liquid surface inside the battery container. As the method for visually recognizing the liquid surface, the following two methods can be mainly used.
In the first method, the liquid surface is visually recognized on a side surface of the battery container. In this case, water is added using, as references, an upper-limit line and a lower-limit line that are indicated on the side surface of the battery container and that indicate the upper-limit level (UPPER LEVEL) and a lower-limit level (LOWER LEVEL), respectively, of the liquid surface. When the liquid surface is lower than the lower-limit line (the electrolyte amount is small), water is added such that the liquid surface will not pass the upper-limit line.
In the second method, a sleeve (liquid surface instructing device) extending from the liquid port to a prescribed liquid surface height of the electrolyte is used (see NPL 1). The lower end of the sleeve corresponds to the upper-limit level of the liquid surface. If the liquid surface does not reach the lower end of the sleeve when the liquid surface is viewed from the liquid port, water is added until the liquid level reaches the lower end of the sleeve. When the liquid surface reaches the lower end of the sleeve, the liquid surface is raised by surface tension, and the upper surface of the electrode plate group immersed in the electrolyte appears to be distorted through the electrolyte. Consequently, one can know that the liquid surface has reached the lower end of the sleeve.
In order to accurately perform adding water, accurate visual recognition of the position of the liquid surface inside the battery container is important. To enhance the visibility, PTL 1 proposes adding a colorant or a fluorescent substance to the electrolyte. PTL 2 proposes liberating the carbon added to the negative electrode.